It is already known to utilize digital transmission channels, intended for example for telephony and operating with pulse code modulation, for transmitting data information, not only if the bit rate in the data information consists of pure submulitples of the PCM channel, i.e. 64 kbits/s, but also for the data rates standardized by CCITT. Such a modulation will be apparent from the Swedish Pat. No. 376 137, describing an example of the transmission of data information at 2.4 kbits/s on a standard 64 kbits/s voice transmission channel.